1 8 Geology and Physiography 



systems that enter the fens from the surrounding upland, and been 

 maintained by poor drainage gradients towards the sea. On the seaward 

 side, the fenland deposits are sUts and clays laid down under conditions of 

 greater or less salinity (see Fig. 47). Alterations in the former relative levels 

 of land and sea have left their trace in the disposition of the two types of 

 deposit. During a phase of marine transgression the silts and clays extended 

 inland above the peat beds, and in the ensuing phase of regression, peat 

 extended seawards over the silts and clays. In this way, the silts and clays 

 from the seaward side, and the peats from the landward side, interdigitate 

 with one another. 



A much simplified scheme showing the relations of the chief fen-beds 

 to one another is shown in Fig. 6. The history of formation of the fen 

 deposits can be very briefly outlined as follows : 



The Pre-Boreal Period. Say before 7500 B.C., a period of sparse birch-pine. 

 Peat was forming on the present floor of the North Sea, the coast of which 

 is now about 200 ft. below its former level. No deposits of this age are yet 

 known from the Fenland. 



The Boreal Period. From about 7500 to 5500 B.C., a period of birch-pine 

 woods, but with oak and elm and hazel in increasing importance. In the 

 deep river valleys of the Fenland peat-formation began during tliis period 

 (e.g. in Little Ouse Valley), and at a few sites with local water supply. 

 During this time the North Sea reached most of its present extent, but it 

 did not directly affect the Fenland. 



The Atlantic Period. From about 5500 to 2000 B.C., a warm wet climatic 

 period marked by the sudden onset and subsequent importance of the 

 alder and, to a smaller extent, of the lime. 



At the beginning of this period, Mesolithic (Tardenois) man occupied 

 local sand liills in the fens, but, very soon, peat-formation became wide- 

 spread throughout the fen basin, and thick beds of fen sedge-peat and fen 

 brushwood-peat were formed. Towards the end of tliis time the peat 

 surface dried and became increasingly wood-covered, partly due, no 

 doubt, to marine recession and partly to climatic dryness. There is some 

 trace of Neohthic man at this time. 



The Sub-Boreal Period. About 2000 to 500 B.C. At the end of the Neolithic 

 or in the early part of the Bronze Age, an extensive but shallow marine 

 invasion caused silt and clay to spread far inland over peat. Foraminifera 

 and diatom analyses suggest shallow brackish lagoon conditions ; this was 

 the stage of formation of the Fen Clay. 



